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BACKGROUND We hypothesized that the combination of a preoperative and a postoperative scoring system would improve the accuracy of mortality prediction and therefore combined the preoperative 'additive EuroSCORE' (European system for cardiac operative risk evaluation) with the postoperative 'additive CASUS' (Cardiac Surgery Score) to form the 'modified CASUS'. MATERIAL AND METHODS We included all consecutive adult patients after cardiac surgery during January 2007 and December 2010 in our prospective study. Our single-centre study was conducted in a German general referral university hospital. The original additive and the 'modified CASUS' were tested using calibration and discrimination statistics. We compared the area under the curve (AUC) of the receiver characteristic curves (ROC) by DeLong's method and calculated overall correct classification (OCC) values. RESULTS The mean age among the total of 5207 patients was 67.2 ± 10.9 years. Whilst the ICU mortality was 5.9% we observed a mean length of ICU stay of 4.6 ± 7.0 days. Both models demonstrated excellent discriminatory power (mean AUC of 'modified CASUS': ≥ 0.929; 'additive CASUS': ≥ 0.920), with no significant differences according to DeLong. Neither model showed a significant p-value (

Background: Prolonged intensive care unit (ICU) stay is a predictor of mortality. The length of ICU stay has never been considered as a variable in an additive scoring system. How could this variable be integrated into a scoring system? Does this integration improve mortality prediction? Material/Methods: The 'modified CArdiac SUrgery Score' (CASUS) was generated by implementing the length of stay as a new variable to the 'additive CASUS'. The 'logistic CASUS' already considers this variable. We defined outcome as ICU mortality and statistically compared the three CASUS models. Discrimination, comparison of receiver operating characteristic curves (DeLong's method), and calibration (observed/expected ratio) were analyzed on days 1-13. Results: Between 2007 and 2010, we included 5207 cardiac surgery patients in this prospective study. The mean age was 67.2 +/- 10.9 years. The mean length of ICU stay was 4.6 +/- 7.0 days and ICU mortality was 5.9%. All scores had good discrimination, with a mean area under the curve of 0.883 for the additive and modified, and 0.895 for the 'logistic CASUS'. DeLong analysis showed superiority in favor of the logistic model as from day 5. The calibration of the logistic model was good. We identified overestimation (days 1-5) and accurate (days 6-9) calibration for the additive and 'modified CASUS'. The 'modified CASUS' remained accurate but the 'additive CASUS' tended to underestimate the risk of mortality (days 10-13). Conclusions: The integration of length of ICU stay as a variable improves mortality prediction significantly. An 'ICU-day' variable should be included into a logistic but not an additive model.